1. Technical Field
This invention is related generally to photogrammetry and more specifically to large format digital cameras for acquiring images of large area objects or large areas of large objects, such as aerial images of portions of the surface of the earth.
2. Background Art
It is often desirable to photograph or otherwise acquire digital images of large areas of objects, but in large formats to enhance geometric resolution, i.e., ability to distinguish geometric details for visual inspection or analysis. For example, it is easy to get a picture of a mountain with a digital camera by backing far enough away from the mountain until the entire mountain fits within the field of view of camera lens, set the focus at infinity, and push the button on the camera to take the picture. The resulting picture of the mountain, which, when processed and printed, can be held in a person's hand and pasted onto a page in a scrap book, may appear to be sharp and clear, but it may not be possible to discern an individual house built on the side of the mountain in the picture because the house is too small. Expanding or “blowing up” the picture to a larger size might make it possible to discern the house, but it may still be impossible to see the doors, windows, roof lines, and other details of the house because the resolution of such fine details just might not be in the picture.
If the camera could have a larger field of view, such as a wider angle lens, so that it would not have to be backed so far away from the mountain to fit the view of the mountain within the field of view of the lens, some enhancement of details, i.e., higher geometric resolution, could be obtained. Such wide angle lenses, sometimes called “fisheye” lenses, are well-known, but they also distort the image—magnifying portions in the middle and diminishing marginal portions of the image.
Geometric resolution obtainable by digital cameras is also limited by the sizes of photodetector arrays. Images are detected in digital cameras by focusing light of an image onto an array of photodetector devices, such as charge-coupled devices (CCD's). Each photodetector in the array produces an electric signal, the voltage or current of which is a function of the intensity of all the light that is focused onto that photodetector. Thus, if all of the light reflected by the house in the above-described mountain scene is focused onto one photodetector, the electric signal (voltage or current) level produced by that photodetector will be indicative of the average light energy reflected from the entire house. Therefore, such signal output might enable one to discern that there is something in the resulting mountain picture where the house is located, but it would not be able to produce a recognizable house shape, much less details of doors, windows, roof lines, and the like. Furthermore, no amount of enlargement or “blowing up” of the picture could produce more detail. It would just produce a larger spot in a larger image of the mountain scene, because the resolution is limited to a single electric signal for the entire house.
On the other hand, if the light reflected from the house could be focused onto an array of tens or even hundreds or thousands of individual photodetectors, so that light reflected from a window, for example, is focused on one or more individual photodetectors, while light reflected from doors, walls, and different roof sections of the house could be focused on other respective multitudes of individual photodetectors, then individual electric signals indicative of light energies reflected from each of those various parts or features of the house would be available for producing more details of the house in the image, i.e., there would be higher geometric resolution in the picture.
These same principles apply to aerial and other types of digital photography. For example, if in taking an aerial picture of a city, all of the light reflected from a city block is focused on one photodetector, it may be possible to discern the block in the resulting picture, but it would not be possible to discern individual buildings, back yards, alleys, cars in driveways, and the like. However, focusing the light reflected from the city block onto an array of hundreds or even thousands of photodetectors would enable geometric resolution capable of discerning such details in the city block.
A problem faced by digital camera manufacturers is that large photodetector arrays, to the extent they are commercially available, are not inexpensive. Therefore, most camera designers and manufacturers are limited to commonly available photodetector arrays. Rectangular array sizes of about 1,000×1,000, i.e., about 1,000,000, photodetectors are now fairly common and readily available, while arrays of 5,000×5,000, i.e., about 25,000,000 photodetectors, or more are considered to be “large” and expensive. These array sizes are impressive compared to only a few years ago, and sizes of photodetector arrays will no doubt continue to grow in coming years. They are, none the less, still limited. Several high-end camera manufacturers do make digital cameras with 10,000×10,000 arrays, i.e., about 100,000,000 photodetectors, but the costs of such cameras are prohibitive fore most purposes. Therefore, most aerial photographers have to choose between setting their digital camera equipment to obtain pictures or images of large areas, such as entire cities or more, with low geometric resolution (i.e., ability to discern few details), or to obtain smaller area pictures or images, such as parts of cities, but with higher geometric resolution.
Besides cost, there are other limitations of such “large” array digital camera, especially as compared to analog cameras using film, which can generally achieve better geometric resolution than conventional digital cameras. For example, radiometric resolution, i.e., gray scale and/or color resolution, of analog film cameras generally not as good as that of digital cameras. On the other hand, large arrays of photodetectors often have defective detector elements that produce defective images. Also, their large size, and, especially the time it takes to read an image of a large array of photodetectors, are all significant problems. The read-out time alone can reduce their usefulness to situations in which there is no movement between the object and the camera, because such movement before all the electric signals from all the photodetectors in the array that can be read could introduce distortions in the resulting picture or image. Further, such large detectors for large format photogrammetry applications have to be nearly flawless, i.e., few, if any, bad photodetectors, because there is no redundancy, and bad photodetectors in the array will leave flaws in images being acquired by the detector array. The requirement for such flawlessness puts further pressure on cost of such cameras.
Other methods to overcome these limitations for acquiring larger format images of large area objects, such as macro- and micro-scanning cameras, also take time to create the image. Scanning cameras are ones in which a linear array of photodetectors is moved in the stationary image plane of a camera to cover the entire image format or view. A similar effect, but different method, is to move (rotate) the camera, usually in an arc about an axis that extends through the lens and is parallel to the scene or object and to the image plane. Cameras that use this latter technique of scanning the object by rotating the camera are called panoramic cameras. Both scanning cameras and panoramic cameras, as described above, also require that the object does not move in relation to the location of the camera, and vice versa, while the object is being scanned to collect the image.
Satellite imaging of the earth's surface or of surfaces of other planets, moons, and the like, has long come to rely on kinematic imaging with a scanning linear detector array, which has come to be known as “push-broom” scanning. In such “push-broom” scanning, the entire camera, including both the lens and the linear array of photodetectors, is mounted on the satellite and moves with the satellite in relation to the object, which is the earth's (or other planet or moon) surface. The image of the object (earth or other planet or moon surface), therefore, is collected in individual linear paths corresponding to the linear array of photodetectors being swept over the object, and a plurality of such paths are then assembled together to produce the image. In such “push-broom” scanning, the object (earth, etc.) must not “move” as it is being scanned, i.e., the satellite with the camera remains a fixed distance from the center of mass of the earth during the satellite's movement and scan of the earth's surface. The problem with such “push-broom” scanning is that it is difficult to know the camera path accurately, which results in a notorious lack of accuracy in stereo measurements or mensuration in the resulting images.
A recent aerial camera system developed by LH Systems, of Englewood, Colo., USA (now Leica Geosystems, of Atlanta, Ga., USA), which is similar to a 1969 doctoral dissertation of E. Derenyi and based on a system previously developed by the German Aerospace Establishment DLR for use on a Mars mission, uses multiple linear detector arrays, instead of film, in the image plane of a conventional high quality lens system to match or exceed performance of analog film-based cameras in aerial photogrammetric applications. As the aircraft or spacecraft flies over the object, each detector array produces a “push-broom” strip of imagery, but these “push-broom” strips are not geometrically independent, since the multiple image lines are collected simultaneously with the same camera. A point on the ground in the path of the multiple “push-broom” image acquisitions is imaged as many times as there are linear arrays in the camera. A problem is that each image line has a separate perspective center, so the resulting composite image of the object has many perspective centers.
A variation of this multiple linear array approach is shown in the Patent Cooperation Treaty (PCT) patent application no. PCT/DE00/01163 (International No. WO 00/66976) filed by W. Teuchert and W. Mayr, wherein the linear arrays are replaced by a plurality of small rectangular arrays to populate the focal image. Three groups of multiple detectors in each group are used, so that one group is forward-looking, one group is nadir (looking straight down), and the third group is rearward-looking. This camera is also mounted on an airplane or spacecraft, and, as it flies, the image taking is repeated rapidly so that spaces between images acquired by individual detectors in a group are filled or partially filled by images acquired by the next row of detectors in the group, etc., until the three group images are filled and synthesized together from the individual detectors in the group. This system requires the motion of the airplane or spacecraft, and it cannot be used to image a moving object. It also has the problem of multiple perspective centers.
Another approach to aerial imaging was announced recently by ZI, Inc., a subsidiary of Intergraph Corp. of Huntsville, Ala., USA, combines several cameras that take images simultaneously. Four cameras, i.e., four separate lens and sensor assemblies, mounted together produce four separate sub-images of the parts of the object that are in the respective fields of view of the four cameras. Each sub-image has its own perspective center, with each of the four optical centers being in the middle of its respective sub-image. A large-image format can be assembled from the sub-images, but accuracy can suffer because of differences among the multiple cameras, and it has the problem of multiple perspective centers—one for each camera.